Digital subscriber line (DSL) technology, often referred to as “broadband”, is a family of services that provides high speed digital data transmission over the metallic twisted copper pairs that form part of a local telephone network. DSL is commonly used to provide a customer's home with a network connection, typically to the Internet via an Internet Service Provider (ISP).
However, DSL lines are susceptible to interference, causing transmission errors and synchronisation problems. Synchronisation on a line can be lost when interference is so large that the data (signal) being transmitted cannot be properly processed. Line management techniques can help address this problem by trying to stabilise a line, usually at the expense of the line synchronisation rate. One such technique is known as Dynamic Line Management DLM.
DLM monitors line parameters (such as error rates and re-synchronisation events) associated with the DSL lines, and sets a line profile for each line accordingly. The line profile determines the line configuration parameters used on a line, which can include the setting of target signal to noise ratio margin, use of error correction techniques, and even capping the line synchronisation rate. All these impact on the rate at which the line will synchronise at. With DLM, use of line profiles provides a trade-off between line stability and synchronisation rate. For example, setting a line profile with a higher target signal to noise ratio margin may result in a more stable line that is more tolerant to the presence of noise on the line, but the resulting synchronisation rate will be reduced.
However, certain conditions can occur in the network where even DLM is unable to find a line profile that can stabilise the performance of the DSL line. Examples of these conditions include Repetitive Electrical Impulse Noise (REIN), and metallic faults on the line such as high resistance joints or unstable physical connections. REIN is particularly problematic as is it is often difficult to identify the source, though it can for example originate from faulty external power sources such as power supply units and lighting units. These conditions can be brief events lasting only seconds, or can be present for longer periods (minutes or even hours). During this time, even with DLM active, the DSL line may not be able to achieve synchronisation or can only do so at a very low synchronisation rate.